High power devices for automotive applications are required to operate at high temperatures and provide for high current operation. The current power train requirements of most hybrid vehicles (HV) and electric vehicles (EV) are met using silicon IGBT (Insulated Gate Bipolar Transistor) devices. Higher performance can be achieved with GaN power transistors based on advantages, such as, lower on-resistance, higher operating temperatures, and smaller systems. However improvements offered by GaN devices are yet to be realized in deployed subsystems.
Several groups of researchers are experimenting and reporting on GaN transistors that are aimed at replacing Si IGBTs. Advantages of GaN devices are summarized in an article by Boutros, Chu and Hughes, entitled “GaN Power Electronics for Automotive Applications”, (IEEE 2012 Energytech—http://toc.proceedings.com/15872webtoc.pdf). As an example, electric propulsion units require high-current (200 to 600 A), high-voltage (100 to 600V), low loss power semiconductor switches. GaN switches are expected to offer ˜100× performance over silicon-based devices, owing to superior material properties such as high electron mobility and high breakdown field and capability to provide GaN power electronics with low on-resistance and fast switching, and higher operating temperatures (Roberts, “Lateral GaN Transistors—A Replacement for IGBTs in Automotive Applications”, PCIM Europe 2014; International Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management; Proceedings of; 20-22 May 2014).
At present, very few high-voltage GaN devices for automotive applications are available on the open marketplace. However, as these devices reach maturity, it is expected that GaN power switches will be introduced into the automotive market for a number of applications such as power generators, power conversion units and electronically controlled on-demand accessories.
For safe operation, normally-off GaN devices with high current and voltage capability are required. Normally-off operation may be provided by series connecting a normally-on GaN transistor with a driver MOSFET in cascode configuration. Alternatively, an enhancement mode (E-Mode) normally-on GaN transistor may be used.
A cascode structure can use a conventional MOSFET or a custom structured MOSFET to provide a threshold voltage that approximates to or has an advantage over an IGBT device, e.g. ˜3V and ˜5V for Silicon SJ MOSFETS and Silicon IGBT devices respectively. Alternatively, a normally-off E-Mode GaN transistor can be used. However, E-Mode GaN devices generally have very low threshold voltages, typically 1.5V or less. This poses a significant problem for safe operation, particularly with respect to noise issues and parasitic elements which could cause voltage spikes or noise in excess of the threshold voltage, thus unintentionally turning on the device. Clearly this is a safety hazard for high current and high voltage applications, such as, automotive applications. While it would be desirable to have threshold voltages of 3V or more for high power applications, currently, few vendors offer E-Mode devices with threshold voltages above 2V, and devices with threshold voltages above 3V are rare. Thus, to manage transients caused by noise issues and parasitic elements and ensure safe operation, low threshold voltage E-Mode GaN transistors require carefully designed driver circuitry with signal isolators, isolated +VE and −VE power supplies and a source-sense Kelvin connection.
In addition to considering the potential for noise to cause false switching, another issue for safe operation is the Miller capacitance effect. The latter could result in the power transistor being turned back on when the gate is being taken low.
Drivers for cascode GaN devices are disclosed in co-pending PCT International patent application No. PCT/CA2013/001019, entitled “Devices and Systems Comprising Drivers for Power Conversion Circuits”, filed 13 Dec. 2013, claiming priority from U.S. provisional patent application No. 61/740,825 filed 21 Dec. 2012, of the same title, and U.S. patent application Ser. No. 14/105,569, entitled “Devices and Systems for Power Conversion Circuits”, filed 13 Dec. 2013, claiming priority from U.S. provisional patent application No. 61/740,821, filed 21 Dec. 2012, of the same title. The use of discrete components and separate driver circuits necessitates interconnection of the components by wire-bonding or other interconnect technologies, which introduces unwanted inductance. Co-packaging of a GaN power device and MOSFET driver circuit also requires effective thermal management. These patent applications disclose driver circuits and packaging arrangements for a cascode configuration GaN device, which seek to address one or more issues of thermal management, series inductance and resistance, to reduce or manage unwanted noise and voltage transients, and enable lower cost and more compact systems and devices for electronic power conversion circuits.
However, for E-Mode GaN devices with lower threshold voltages, the use of discrete driver circuitry poses even more significant challenges in managing these issues with noise and parasitic elements, including Miller capacitance effects. Accordingly, there is a need for improved solutions using integrated drivers for E-Mode GaN devices.
The Miller Ratio (QGD/QGS) provides an indication of how sensitive a switching transistor is to false, unwanted switching. As the rated voltage increases the GaN transistor Miller Ratio degrades. Thus, higher voltage GaN transistors are more susceptible to false, unwanted transient operation than low voltage GaN transistors.
The need to overcome these driver difficulties was recognized some years ago, at the time small low voltage GaN transistors were first introduced, for example, as disclosed in a presentation by A. Lidow entitled “The GaN Journey Begins”, IEEE SCV Electron Devices Society (EDS), Oct. 12, 2010.
The following references, and other references cited therein, provide further background information on drivers for GaN FETS:                a) Texas Instruments Inc. Datasheet LM5113 5A, 100V Half-Bridge Gate Driver for Enhancement Mode GaN FETs (SNVS725F—JUNE 2011—REVISED APRIL 2013);        b) U.S. Pat. No. 8,593,211 to Forghani-Sadeh (Texas Instruments Inc.) entitled “System and apparatus for driver circuit for protection of gates of GaN FETS; and        c) U.S. Pat. No. 8,766,711 to Takemae (Transphorm Japan Inc.) discloses integrated drive circuitry.        
There is a need for further improvements in driver circuitry for E-Mode GaN devices, particularly for power switching systems comprising high power e-mode GaN transistors with integrated driver circuitry.